1. Field of the Invention
The present invention pertains to highway sound barriers whose inside walls have multiple faces tilted to aim reflected sound energy away from populated areas.
2. Description of Prior Art
Since the passage of the Federal Noise Control Act of 1972, increased effort has been placed on minimizing the amount of highway noise, or unwanted sound, that is transmitted to nearby residential areas. The choice of type of sound barrier is usually based on relative costs. Because of concrete's versatility and low material cost, many states choose vertical sound barriers that are constructed of prefabricated concrete panels or sub-panels which fit tongue and groove into vertical posts, forming vertical walls. Other states prefer to use vertical walls made of wood or concrete block. Since the prefabricated parts are easy to transport and are durable in an outdoor environment, construction and maintenance costs are low. Vertical barriers occupy very little horizontal space across a highway right of way and are easy to install in a wide variety of situations, including sloping terrain and curving roadways.
A three inch thick concrete highway sound barrier has a ASTM sound transmission class (STC) rating of 47 dB noise reduction; however, a pair of barriers 3 to 11 meters (10 to 35 feet) high, placed on opposite sides of a highway, achieve only 7 dB to 15 dB of noise reduction.
Most highway noise heard by people at ground level beyond sound barriers has passed over these barriers rather than through them. As sound energy passes over the sound barrier rays of sound close to the top edge of the barrier will be diffracted downward. Whether they are bent downward enough to be heard at ground level depends on how close the rays are to the top edge, and how nearly horizontal the rays are traveling as they pass over the barrier.
In addition, when downward atmospheric refraction occurs, sound traveling at a low angle to the horizontal will be refracted to the ground. This downward refraction is caused by temperature inversions and wind. In good weather, both are likely to occur during the evening hours as it cools. Unfortunately, in warm weather, that is the time of day that people want to use their back yards the most.
Above 35 miles per hour, the dominant noise generated by highway traffic is tire noise. On a highway, tire noise will occur on all parts of both roadways. Therefore, sound rays, reflected from a corresponding barrier on the opposite side of the highway passing close to the top of a barrier will be travelling in a direction that is more nearly horizontal than rays direct from the road surface.
Concrete and wood sound barriers have very poor sound absorptivity. The Sabine absorptivity for poured, unpainted concrete is less than 0.05 on a scale of 0 to 1 for all frequencies of sound below 4,000 Hz. Wood barriers have an absorptivity index less than 0.1. When sound strikes one of these sound barriers the incident sound energy not absorbed by the barrier is reflected. Because of the low Sabine number of concrete and wood, 90% to 95% of the sound energy that strikes a sound barrier having vertical walls made of these materials will be reflected.
Even though there is a 5% to 10% loss of energy when the sound is reflected, this is compensated for by the lower angle of the reflected sound rays. Because of diffraction, at 150 feet or more beyond the barrier, the reflected sound level will equal or exceed the direct sound level. When two sound sources of equal intensity merge, the total sound level is increased by 3 dB. Refraction can further intensify the sound, amplifying the reflected sound more than the direct sound.
Thus sound barriers with sound reflective vertical walls facing the highway from both sides of the right of way create reflected sound that is counterproductive to the barriers' purpose of reducing the highway noise level in order to preserve the value and enjoyment of a residential area.
Concrete barriers have been installed with their flat panels tilted away from the highway. This will cause sound to be reflected upward but the installation of these panels is more difficult and expensive. The columns must also be installed at an angle. This requires special post hole diggers that can dig deep holes at a slant and two lifting devices to guide the columns into their holes rather than a single crane to lower the columns into vertical holes. If the the columns are cantilevered they have to be longer since deeper footings are required. Temporary outrigger supports are needed while the concrete footings strengthen. The panels are difficult to insert in the grooves of the columns and custom panels are required for curves in the roadway. The tilted walls and any permanent outrigger supports are unattractive from the outer side so this type of barrier is unsuitable for installation when abutting back yards and other populated land areas.